Heat switches are used to control the flow of heat in thermal systems, coupling stages of refrigeration. They are critical components of a variety of refrigerators that operate at very low temperature (e.g., less than 5 kelvin (K)), including adiabatic demagnetization refrigerators and helium-3 and helium-4 refrigerators. One of the most common types is the active gas-gap heat switch. In a conventional active gas-gap heat switch, there are two main components: the switch body and the external getter (i.e., sorption pump). The getter is connected to the main switch body.
The switch body contains two sets of thermal conductors located inside a containment tube that maintains a close spacing between the thermal conductors, and positions the thermal conductors such that their surfaces face each other. Each set of thermal conductors is connected at one end to a flange that can be sealed to the containment tube. The two sets of thermal conductors are inserted and sealed to the containment tube from opposite ends. The flanges provide a means to make thermal contact between external components and the thermal conductors inside the switch body. The enclosed volume is hermetically sealed with a quantity of gas, usually helium for low temperature switches, inside. The external getter is a similarly sealed volume that contains a material with large microscopic surface area that, in the desired temperature range, has a high binding energy for the chosen gas.
The switch body and getter are connected so that they form a single enclosed volume, usually by means of the interconnecting tube. The connection is made in a way that allows the getter to be heated and cooled independent of the switch body. The switch is opened and closed by cooling the getter below or warming it above a threshold temperature. When the getter cools below a threshold temperature, gas empties from the switch body and adsorbs onto the getter, opening the switch. Similarly, when the getter is warmed above the threshold, the gas desorbs and the switch closes. In a typical active gas-gap heat switch, the getter material is charcoal or zeolite, the fill pressure is one atmosphere of helium (either helium-3 or helium-4), the temperatures at which the switch is used are less than ˜5 K, and the threshold temperature for the getter is 12-13 K. Also, typically, the amount of heat needed to warm the getter is on the order of 1 milliwatt.
The connection between the switch body and the getter presents some significant design challenges. The goal is to have a low gas impedance between the getter and the switch body and a high thermal impedance. The low gas impedance allows the gas to flow quickly between the two volumes, especially at the very low pressures needed for the switch to be truly “open.” The low thermal impedance minimizes the flow of heat from the getter when it is warm to the switch body. This flow of heat can be a significant source of inefficiency for low temperature refrigerators using active gas-gap heat switches.
However, these two goals cannot be met simultaneously with conventional switches. Lower gas impedance requires a larger diameter, shorter interconnecting tube, while lower thermal impedance requires a smaller diameter and a longer length. A further goal of any design is for the transition time between open and closed states to be short. This is limited by two factors: (1) the thermal impedance of the interconnecting tube; and (2) the heat capacity of the getter assembly. The first factor arises because, for designs in common use, the only means of cooling the getter is heat flow through the interconnecting tube. In practice, the ratio of heat capacity of the getter to the thermal conductance of the interconnecting tube (conductance is the inverse of impedance) is the time constant for the getter to cool. Hence, a small heat capacity and large, or at least moderate, thermal conductance is desirable. This latter desire is also at odds with the two goals above.
Thus, conventional heat switch designs are not capable of achieving a low gas flow impedance, a high thermal impedance, and a short transition time between open and closed states. Accordingly, an improved heat switch may be beneficial.